Origin and evolution of whales
Introduction
Whales are aquatic mammals. Whales are under the infraorder Cetacea. The two parvorders of whales are; baleen whales and tooth whales. Toothed whales include; dolphins, porpoises. Scientifically they belong to the Animalia kingdom and the class of Mammalia. Whales evolved from land living mammals. Indohysus are the closes sister group to modern whales and did not descend from them. Whales over the time have descended from pakicetid, Ambulocetids, Remingtonocetidae, protocetids and Basilosaurids.
Indohysus which is a small animal that lived about 50 years ago in the northern region of India is believed to be the closest sister group of Cetacea. Indohysus share some traits with modern whales and particularly the Involucrum, which is a bone growth pattern that most whales are associated with and no other species have it. It also showed little signs to adaptation of marine life; signs such as short heavy limbs that that help them stay under water, this adaptation of there is similar to those of hippos. Pakicetids were the earliest whales and were hoofed mammals. They are known to live in the early Eocene around 50 million years that have passed, the first of its remains were found in Pakistan in 1979, at the what would be regarded as the shores of Tethys ocean. Many more discoveries followed. The reason, they are associated with evolution of whales is the skull of its auditory bulla which is commonly referred to as the ear bone, it is highly unusual and resembles that of a whale. Its jaw bones resemble the teeth of fossils whales. Its serrated, triangular in shape teeth also links it to modern whales. They were poor swimmers and this is proved by the fact that they had short hands and feet (Understanding evolution, n.p).
Oxygen isotopes of bones of Ambulocetids are high in salty water. This indicate that these whales lived near marine shore lines and drank salty water like the existing whales. These whales also had nostrils that were positioned further and at the back of their snouts a trend that is seen in modern whales which have nostrils on their heads. These ancient whales were also large in size, they had a fat jaw like what is seen in modern whales. they also had long back hinds and a tail that showed no sign of a fluke. This shows that as whales engaged more in aquatic life the more their nostrils also evolved to be located further away in the back of their skulls. These early aquatic whales continued to show changes that confirm that they are ancestors of modern whales. The pelvis of these early aquatic whales continued to decrease in size (Understanding evolution, n.p). their veritable column also increased.
Remingtonocetidae are also ancient whales, they were found in Middle-Eocene in south Asia 43 million years that have passed. Their habitat was shallow marine deposits although they proved to be more aquatic than ambulocetid, it has been proved by recovery of their remains in shallow marine areas such as near shores and lagoons. According to oxygen isotopes in their bones they did not ingest fresh water nor were they dependent on it. They rarely used their sense of vision since they had small orbits. According to oxygen isotopes of their bone they did not depend nor live in fresh water. Their features do not give any clue if they came to land, compared to all other ancient whale they had shorter limbs. This reduction in size was preparation for the mammals that evolved from it to adopt marine life (Thewissen, 35).
Whales are also believed to have evolved from protocetid, which are a diverse group from Asia, Europe and Africa. They are known to have lived around 43 million years that have passed. Its first remains were discovered in a coastal lagoon in South Asia, they were more aquatic compared to remingtonocetids, and Ambulocetids. They were the first ancient whales to live their habitat and disperse to oceans all over the world. They also displayed great aquatic adaptation, some of them were able to support their weight on land while others could not. Unlike the remingtonocetids and ambulocetids they have big orbits which were oriented literally and are similar to those of modern whales that might have been used to observe under water prey, its nasal opening was large and near the snouts. They had short limbs that were used for swimming. They also had a variety of feeding techniques, they have methods of sound transmission that link them to modern whales although their method of sound transmission was not much advanced like that of modern whales. (Thewissen,36).
As whales begun to swim more changes took place in their skeletons. Their limbs begun being used more for steering than their former use which was padding. The sequence of the tails of these early whales resemble those of living dolphins. Other early whales such as Dorudon and Basilosaurus had tailfins that they developed in order to adjust to their aquatic lifestyles, these two ancient whales are known to have come to existence 40 million years that had passed. The structure of their body allows them to propel themselves forward and resist the force on water as they swim. The hind limbs of these two whales were none existence and it is believed that they served no function in their bodies and may have been internal to the body wall like what is seen in today’s whales, occasionally tiny hind limbs are discovered inside their body walls (Thewissen, Cooper, George, & Bajpai, n.p).
Basilosauridae can be regarded as the first fully aquatic whale around 40 million years ago and they spent their entire life in the ocean. They had a blow hole that was shifted toward their eye, they had a fluke acting as the tips of their tail. Their hind limbs could support if they were to walk on land and had flippers for fore limbs. All these features are responsible for making them similar to modern whales and their features did not allow them to leave the ocean. There were seven genera of this whale but can be classified further into two categories according to their body types; basilosaurus which have a body that resembles that of a snake with an approximate length of 17 m, they swam with sinuous movements taking their entire body along with them. Other body type of this ancient whale is shorter and approximately 4 m short this is was called dorudontines, its shapes is easily recognized as the shape of modern dolphins. Their skeletons are easily recognized and are similar to those of modern whales. despite their similarity to modern whale they were not able to use echolocation like modern whales. Unlike the long one it swam using an up and down motion of their tail fluke (Thewissen, Cooper, George, & Bajpai, n.p).
The resemblance between bisilosaurid ancient whales and modern whales is very visible. As time went by ancient whales went on to evolve, the first modern whales referred to as odontocetes and mysticetes were discovered 34 million years that have passed. They descend from a cetacean known to be monophyletic. One of the most famous innovation of odontocetes whales is echolocation that alerted them of what was surrounding them, echolocation would later prove to be important to modern whales in hunting. A new feeding method was adopted by these whales it involved filtering food using baleens. Filter feeding and echolocation are important to modern whales. These sea mammals that resembled modern whales are known to have conquered nearly all the oceans. Deep seas, shallow seas, riverine, tropical and artic waters all inclusive (Thewissen, Cooper, George, & Bajpai, n.p)
During the time of Darwin, the only known and oldest cetacean was basilosaurids which was large and streamlined in shale and are easily recognizable to anyone who is familiar with modern whales. A hundred and thirty years later the Pakicetids remains were discovered. When the oldest basilosaurid remains were discovered they were not identified as the remains of a whale. Whales are cetaceans and some of their features are indicators of their land ancestry. They are often on the surface of water breathing air after some time, even though their bodies have fully adopted living in water. Traces of the land ancestry of whales is visible in the features of their embryos. At the first stage of these embryos they have hind libs that are visible externally but as it continues to evolve the hind limbs disappear gradually (Thewissen, 42).
After scientist discovered that whales’ ancestors were land mammals, they sought to discover mammals that were related to them. A whale being a cetacean it proved hard to find any relationship between it and other land mammals, the fact that hippopotamuses are their close relative has been contradicted by the fact that whales date 50 million years back in history and there is no possibility that their closely related relatives are alive. Paleontologists continue to look for the land even-toed ungulates mammal that displays any relationship to the whales. In 2007 skeletons of raoellids were found, these skeletons display a close relationship to the whales when studied (Thewissen, Cooper, George, & Bajpai, n.p).
Modern whales are categorized as baleen whales and toothed whales. All modern baleen whales have baleens in place of teeth although the use of baleens is different among the species. Filter feeding is another beneficial factor in the lives of baleen since it is their way of feeding. The development of filter feeding can be credited to the environmental change and physical changes that took place in the ocean as time went by. The ancient baleen whales had very little or no baleen and relied heavily on their teeth. Whales were forced to adapt to echolocation when toothless whales split from baleen whales, this happened about 34 million years that have passed the technique of echo location helps them locate their prey while hunting. The skulls of an ancient whale, squalodon were the first to show evidence of echolocation and lived around 33 million years ago, however, it is unlikely that they are the ancestors of toothed whales (Thewissen, Cooper, George, & Bajpai, n.p).
Conclusion
Evolution of whale dated beck up to 50 million year ago, when Indohysus showed features that resemble those of modern whale. Hippos have been known to be the closest relative of whales that exist to date. Pakicetid were the earliest whales, they were hoofed mammals. Its first remains were discovered in Pakistan in 1979 near a river that was close to the ocean Tethys. The skull of its auditory bulla resembles that of modern whale. Amblulocetus is another ancient whale although its features were far more developed to resemble this of modern whales compared to pakicetid, they survived more in marine water and depended on it according to oxygen isotopes of its bones. Remingtonocetidae is also an ancestor of modern whales. They had notable features such as reduced semicircular canals that were responsible for helping them balance on land. Basilosaurids were the first whales to live in water only and its close relationship to the modern whales is very visible. odontocetes and mysticetes were discovered 34 million years that have passed and echolocation that helped them navigate their surrounding them hunting was noted. As whales continued to evolve so did their feeding techniques, baleen whales begun filtering their food using baleens located in their mouths. Raoellids have been named as the closest relatives to whales. The origin and evolution of whale has become one of the most and best documented macroevolutions.
References
Thewissen, J G. M. H. Walking Whales - from Land to Water in Eight Million Years., 2014. Print.
Understanding evolution. The evolution of whales. n.d. retrieved from; https://evolution.berkeley.edu/evolibrary/article/evograms_03
Thewissen, J.G.M., Cooper, L.N., George, J.C., & Bajpai, S. From Land to Water: The Origin of Whales, Dolphins and Porpoises. (2009). Retrieved from; https://link.springer.com/article/10.1007/s12052-009-0135-2
